1. Field of the Invention
The present invention relates to a transmission system for a vehicle and more particularly to an automatic transmission whose gear trains are originated from those of a conventional manual transmission.
2. Discussion of Prior Art
Generally, a manual transmission, in which the gear is manually shifted, has an input shaft directly connected to an engine and having a plurality of drive gears and has an output shaft having a plurality of driven gears paired with the drive gears and connected to drive wheels. That is, there are provided a plurality of shift gear trains between the input shaft and the output shaft. When gear is changed, after a clutch is disengaged, changeover mechanisms such as synchromesh mechanism are manually operated to change over paired shift gear trains and then the clutch is engaged. This sequence of manual operations accomplishes a gear shift of a vehicle.
The manual transmission can be converted into an automatic transmission by replacing the sequence of those manual operations with automatic operations using hydraulic actuators. This type of automatic transmission has advantages such as a small number of components, a good transmission efficiency of power and the like, compared to a conventional automatic transmission primarily constituted by planetary gears, friction engagement elements (clutches, brakes) and the like.
This type automatic transmission having a plurality of shift gear trains is-called Automated Manual Transmission (hereinafter, referred to as xe2x80x9cAMTxe2x80x9d). Japanese Patent Application Laid-open No. Toku-Kai 2000-55184 discloses an AMT including a main clutch (dry type clutch) for changing over the connection of a crank shaft with an input shaft between an engagement condition and a disengagement condition and a bypass clutch (hydraulically operated multiple disc clutch) for transmitting torque from the input shaft to an output shaft to prevent a so-called xe2x80x9ctorque dropxe2x80x9d. When the main clutch changes an engagement condition, the bypass clutch is engaged by hydraulic pressure so as to prevent an abrupt drop of output torque when the gear is shifted and thus a shift shock can be alleviated.
An AMT having a bypass clutch has an advantage that an abrupt drop of output torque can be prevented by the engagement of the bypass clutch at gearshifting. Time for switching over power through the bypass clutch at gearshifting is preferably as short as possible. In particular, when the gear is up-shifted during high speed revolution of the engine, it is necessary to synchronously engage a drive gear with a driven gear and reduce the engine speed as fast as possible and as accurately as possible in order to smoothly change over gear trains doing power transmission by the changeover mechanism. That is, in order to accomplish the shift operation swiftly, it is necessary to accurately reduce the revolution of the input shaft, or the engine speed up to a synchronous revolution speed. However, it is difficult to reduce the engine speed swiftly and accurately with the bypass clutch and the engine control using the electronic control throttle valve.
Hence, Japanese Patent Application Laid-open No. Toku-Kai-Hei 4-203669 discloses a technology in which a brake is mounted on the input shaft for the purpose of preventing an over-revolution of the engine when the gear is up-shifted and a synchronizer clutch is operated when the gear is down-shifted.
However, in this technology, since a clutch is released to disconnect power transmission from the engine to the input shaft at gearshifting, particularly, when the gear is shifted from the 1st speed ratio to the 2nd speed ratio or from the 2nd speed ratio to the 3rd speed ratio, the change of driving force is so large that a shift shock can not be eliminated.
It is an object of the present invention to provide an AMT type automatic transmission system having a bypass clutch capable of changing gears swiftly and smoothly.
To achieve the object, the automatic transmission system having an input shaft, an output shaft, a plurality of drive gears mounted on the input shaft, a plurality of driven gears mounted on the output shaft and meshing with the drive gears, a torque converter disposed between a crankshaft of an engine and the input shaft, synchromesh mechanisms for synchronously engaging the drive gears with the driven gears and a shift controller for automatically actuating the synchromesh mechanisms so as to obtain a required gear ratio, comprises a lock-up clutch incorporated in the torque converter between the crankshaft and an output element of the torque converter for connecting the crankshaft with the input shaft, an electronically controlled throttle valve for automatically operating to reduce a rotation speed of the crankshaft when the gear is shifted so as to smoothly synchronize the drive gears with the driven gears, a bypass clutch for transmitting torque from the input shaft to the output shaft when the gear is shifted, while the electronically controlled throttle valve operates to reduce a rotation speed of the engine, an input clutch disposed between the output element of the torque converter and the input shaft for selectively controlling a torque transmission from the output element of the torque converter to the input shaft when the gear is shifted, and a brake mechanism disposed on an impeller shell of the torque converter for additionally reducing a rotational speed of the crankshaft while the electronically controlled throttle valve operates to reduce a rotational speed of the crankshaft so as to smoothly and swiftly engage the drive gears with the driven gears.